Recent developments in telecommunication technology have led to passive optical telecommunication systems at the level of the subscriber lines wherein a respective plurality of decentralized equipment (subscriber stations or what are referred to as distant units respectively combining a plurality of subscriber stations) are respectively connected via a separate light waveguide line to an optical brancher that is connected to a common light waveguide terminal of a centralized equipment (particularly represented by a switching center) via a light waveguide bus. They are connected thereto either directly or via at least one further optical brancher (see European reference EP-A-0 171 080; ISSLS '88, Conf. Papers 9.4.1 . . . 5; and BR Telecom Technol. J. 17(1989)2, 100 . . . 113).
In such a passive optical telecommunication network, the signal transmission proceeding from the centralized equipment downstream to the decentralized equipment can occur in the TDM cell stream from which each decentralized equipment takes only the cells intended for precisely this decentralized equipment. The signal transmission from the decentralized equipment upstream to the centralized equipment can be carried out in a TDMA method, in accord wherewith a decentralized equipment transmits each burst synchronized using a delay means individually set apparatus-associated by the central equipment (see European reference EP-A-0 460 398).
The signal transmission downstream proceeding from the centralized equipment to the decentralized equipment and the signal transmission proceeding from the decentralized equipment upstream to the centralized equipment can thereby be carried out in one and the same wavelength window (for example, in wavelength common frequency mode at 1.3 .mu.m).
The introduction of new broadband communication services is quite generally dependent on the nature and scope of the telecommunication infrastructures that are already present together with the telephone communication services offered therein and on the demand for broadband telecommunication possibilities. The potentially greatest volume of connections can be seen in the area of private households. This potential of connections, however, will not be reflected in an effective demand for connections without appropriate low costs of a broadband subscriber terminal.
In order to enable a subscriber to use broadband ISDN services (for example, interactive video calling video on demand (VoD), teleshopping, information searching, and narrow band services such as (N-) ISDN or traditional telephony (POTS)), various connection possibilities are currently being considered. Solutions wherein infrastructure can be used that already exists are especially attractive. For example, the coaxial cable networks of the CATV operators represent an appropriate medium. The frequency range of, for example, 50 through 450 MHz is utilized by conventional, analog signal television channels; the range below and above the analog signal telephone distribution has been free up to now and can be exploited for new services. A sub-range of the range that was free is being used for what is referred to as cablephone in the United States by some cable TV companies. Other operators are envisioning a comprehensive system that offers a majority of the aforementioned services within the framework of an access network, for example on an ATM basis (AN/A), whereby an optical offering line (fiber feeder) must usually precede the coaxial sub-networks because of the limited range (TELEPHONY, 1 November 1993, 48 . . . 53).
A further, passive optical network (PON) for bi-directional, interactive, switched telecommunication has already been utilized (Der Fernmeldeingenieur 46(1992)10, FIG. 11.2--System OPAL 4), in addition to a passive optical network (PON) with expansion by a coaxial line tree network for unidirectional distribution of communication (TV).
According to an even more advantageous fashioning of a subscriber line network having coaxial line tree networks respectively shared in common by a plurality of subscriber-side line termination units and having light waveguides that connect these coaxial line tree networks to connection equipment, the coaxial line tree networks can be connected via a respective converter equipment to a light waveguide tree network containing optical brancher. This light waveguide tree network is designed both for bi-directional telecommunication services, preferably in bidirectional wavelength division mode, as well as, unidirectional distribution communication services. This enables an extremely economical coverage of a great number of subscribers both with distribution communication services as well as with interactive, switched telecommunication services.
The light waveguide tree network can thereby be a passive optical network or can be an active optical network provided with amplifiers or can also be a simple point-to-point connection. Regardless thereof and independently of one another, the individual coaxial line tree networks can be amplifier-free passive coaxial line tree networks or active coaxial line tree networks provided with amplifiers. This flexibility also enables the realization of networks having substantially different ranges. The bi-directional or, respectively, interactive telecommunication services can be implemented with digital signals transmitted in the form of ATM signals.
In bidirectional digital signal communications, the bit rates from and to the subscriber can differ. The bit rate required by a subscriber can also be individually assigned to each subscriber. The bit rates also do not need to be uniform on the individual coaxial line networks. On the contrary different transmission bit rates for the two transmission directions can also be provided on each coaxial line network. The bit rates of the two transmission directions can also differ on the light waveguide tree network. They can be selected independently of the bit rates of the coaxial line tree networks. When light waveguide tree network and coaxial line tree network use different bit rates, then a rate matching occurs in the converter equipment lying between light waveguide tree network and coaxial line tree network.
FIG. 1 schematically shows an example of such a subscriber line network. Coaxial line networks CN having the standard tree structure are indicated in the right-hand part of the drawing. At the subscriber's side, the coaxial lines are respectively terminated with a network termination means NT/A which is capable of converting its reception and transmission signals such that the connection of standard terminal equipment is possible. A network termination means NT/A has, for example, terminals for distributed television or video on demand, for traditional telephony (POTS) and/or narrow band ISDN, or for any desired broadband ISDN service as well.
At the side facing away from the subscribers, the coaxial line networks CN are respectively connected via an opto/coax converter equipment (Optical/Coax-Converter) OCC to an optical central office line of a light waveguide network OB branched via optical branchers (Splitters) V that connects the coaxial line networks to a connection unit CU/A preferably formed with an (ATM) cross-connect. As also indicated in FIG. 1, a plurality of such light waveguide networks OB can be connected to such a connection unit CU/A. Correspondingly, as likewise indicated in FIG. 1, a plurality of, for example, up to four coaxial line sub-networks CN can be connected to a converter equipment OCC. When a coaxial line network CN enables the connection of, for example 100 subscribers, the converter equipment OCC services, for example, 400 subscribers and the connection unit CU/A services, for example, 2000 through 4000 subscribers.
The transmission in the subscriber line network can be undertaken on the basis of ATM cells (cell-based) with a system-specific overhead. The data rate in the transmission direction toward the subscriber (downstream) will thereby be greater at, for example, 622 Mbit/s than the data rate in the opposite transmission direction (upstream) with, for example, 155 Mbit/s, whereby the downstream transmission can be undertaken in a plurality of channels (for example, in four channels of 155 Mbit/s each) from a transmission-oriented point of view.
In the system outlined in FIG. 1, analog TV distribution signals are supplied proceeding from a CATV source into the connection unit CU/A shared in common by the connected subscribers and are transmitted to all connected subscribers. At the subscriber these television signals can be received in a standard way by a television receiver TV in FIG. 1 that is connected to the line termination means NT/A.
In addition to the analog TV signals, let digital signals in the ATM format be supplied to the connection units CU/A in the telecommunication system outlined in FIG. 1. For example, such signals can be digital video signals of a video-on-demand service (also including an ATM return channel for the program selection by the TV subscribers) or broadband interactive data signals, whereby the digital video signals are likewise received by the television receiver TV with a corresponding auxiliary device (set top box) that is not separately shown in FIG. 1.
Further, narrow band ATM voice and, potentially, data signals as well can be transmitted in both directions in the system outlined in FIG. 1, this being indicated in FIG. 1 by a telephone connected to the network termination means NT/A. Further services that can require the connection of further terminal equipment to the respective network termination means NT/A are possible without being depicted in FIG. 1.
The ATM signals are conducted via an ATM switching equipment ASN. For video- on-demand, a video server VS is required wherein the video programs (films) to be called in are stored, namely in digital and data-compressed form as a rule, for example according to the MPEG2 algorithm with, for example, 4 Mbit/s produced at the ISO-MPEG. Let the server VS be controlled by a controller VODC that evaluates signalling information coming from the connected subscribers via the respective return channel and correspondingly controls both the program output from the video server VS as well as the ATM switching equipment ASN.
The general employment of the asynchronous transfer mode (ATM) for digital signals of all services (with the exception of the TV distribution service based on analog signal transmission) is extraordinarily advantageous because of its great flexibility. Signals having different, arbitrary data rates can be mixed as desired. A selection of data rates corresponding to specific hierarchy levels is superfluous. This is also extremely interesting particularly for a video transmission since, on the one hand, a universally declared data rate for a video signal does not exists anyway and, on the other hand, different picture qualities can be offered by the selection of different data rates. This is particularly interesting in conjunction with a scalable coding, whereby a high quality, for instance on the basis of an HDTV signal having a high data rate, or a lower quality, for instance with a TV signal having 625 line resolution, can be optionally offered to the subscribers (at different prices).
As is shown in practice, the recovery of a stable clock signal at the receive side is indispensable for the faultless functioning of the receive-side decoder, particularly given digital, data-compressed video signals transmitted in the form of ATM signals. In an ATM system, the signal transmission is fundamentally asynchronously undertaken, i.e., the individual ATM cells in fact arrive in the proper sequence in the receiver but at non-uniform time intervals. The recovery of an adequately stable reception clock is thereby in fact fundamentally possible, for instance by employing a large buffer memory for smoothing the data stream (with the consequence of an undesired signal delay), or on the basis of the co-transmission of regularly mixed-in timing marks, among others. Such measures, however, are complicated and expensive. As already mentioned, a cost-optimized solution specifically at the subscriber equipment is of great significance for the acceptance of the system.